This invention relates generally to radiation detectors and in particular to infrared detectors of the photovoltaic diode type.
Photovoltaic diode detectors presently available usually have limited wavelength signal detection capabilities because of bandgap characteristics of its semiconductor material in a photon-counter mode of operation. Because of the narrow bandgap of the single semiconductor layer involved, diode junction resistance, which is mathematically related to bandgap magnitude, is correspondingly small when long wavelength radiation is being sensed. The signal voltage output of such photodiode is also small as a result of the relationship of the wavelength to the bandgap of the semiconductor material. The small bandgap involved also gives rise to a high noise level in the signal output even in the absence of illumination because of the background components of the radiation being absorbed causing generation of charge carriers transported across the diode junction to create random noise in the process. One example of such a photodiode detector of a single semiconductor layer type is disclosed in U.S. Pat. No. 4,763,176 to lto.
Heterojunction types of multi-layer photodiode detectors on the other hand are disclosed for example in U.S. Pat. Nos. 4,297,717, 4,686,550 and 4,763,176 to Li, Capasso et al. and Ito, respectively. However, none of the latter three patents are specifically concerned with enhancing signal output heretofore degraded when detection of relatively long wavelength infrared radiation is involved.
Accordingly, it is an important object of the present invention to provide an enhanced signal from a photodiode type of infrared radiation detector having a small bandgap semiconductor layer through which long wavelength radiation is absorbed.
An additional object in accordance with the foregoing object is to enable a greater flexibility in the design of photodiode detectors to meet different requirements with unexpectedly high signal output voltages and a reduced signal-to-noise ratio in response to detection of infrared radiation within a relatively long wavelength band.